The recycling of used or discarded tires and other rubber products, which are almost always cured or vulcanized, by devulcanization has proven to be an extremely challenging problem. This problem can be attributed to the fact that vulcanization cross-links polymers in the rubber or elastomer with sulfur. The resultant cross-linked rubber or elastomer is thermoset, thus preventing it from being melted or reformed into other products like thermoplastic polymers or metals.
An increasingly dire need exists to recover the used or discarded tires and other rubber products in uncured, or devulcanized, form: The fossil fuels, e.g., petroleum, natural gas and coal, are the feedstocks for manufacturing various kinds of synthetic rubber and elastomer. They are also energy sources for producing and transporting natural rubber.
A variety of devulcanization processes have been invented to recover or reclaim rubber from the used or discarded tires and other rubber products comprising cured, or vulcanized, rubber or elastomer. The recovered rubber can be co-cured or co-vulcanized with virgin rubber to manufacture new tires and other rubber products if a large-scale devulcanization process can be carried out without degradation of the rubber at a relatively low cost. Nevertheless, no devulcanization process has proven to be commercially viable on a large scale heretofore. This is due to the fact that every devulcanization process invented to date is excessively expensive to construct and operate; moreover, every process is exceedingly difficult to scale up and control, and/or cumbersome to recover and purify the high-quality devulcanized rubber with minimum degradation because of one or more of the following reasons: (1) operating at an inordinately elevated pressure; (2) operating at a very high temperature; (3) being subjected to extremely large shear forces; (4) needing to use expensive vessels and mechanical devices, e.g., extruders and high-speed rollers; (5) requiring to supply a special form of energy, e.g., ultrasonic and microwave radiation; (6) being subjected to a mixture, or composition, of two or more reagents, catalysts and/or promoters, which are frequently highly toxic; (7) requiring an unusually long time even for partial devulcanization of cured rubber or elastomer; and (8) only capable of devulcanizing the surface of reclaimed rubber crumb. Typical or well-known devulcanization processes invented to date, all of which suffer from one or more of these 8 major deficiencies, are outlined in the following.
U.S. Pat. No. 4,104,205 discloses a process to devulcanize sulfur-cured elastomer containing polar groups. This process applies a controlled dose of microwave energy of between 915 MHz and 2450 MHz and between 41 and 177 watt-hours per pound, which is sufficient to break substantially all carbon-sulfur and sulfur-sulfur bonds but insufficient to break appreciable amounts of carbon-carbon bonds.
U.S. Pat. No. 5,284,625 discloses a process that continuously applies ultrasonic radiation to a vulcanized elastomer or rubber to break the carbon-sulfur, sulfur-sulfur and, if desired, the carbon-carbon bonds in a vulcanized elastomer. Through the application of certain levels of ultrasonic amplitudes in the presence of pressure and heat optionally, it is reported that cured, i.e., vulcanized elastomer or rubber, can be broken down. By means of this process, the rubber becomes soft, thereby enabling it to be reprocessed and reshaped in a manner similar to that with previously uncured rubber or elastomer.
U.S. Pat. No. 5,602,186 discloses a process for devulcanizing cured rubber by desulfurization. The process comprises steps of: (1) contacting rubber vulcanizate crumb with a solvent and an alkali metal to form a reaction mixture; (2) heating the reaction mixture in the absence of oxygen and with mixing to a temperature sufficient to cause the alkali metal to react with sulfur in the rubber vulcanizate; and (3) maintaining the temperature below which thermal cracking of the rubber occurs, thereby devulcanizing the rubber vulcanizate. The said patent indicates that it is preferable to control the temperature below about 300° C., or where thermal cracking of the rubber is initiated.
U.S. Pat. No. 5,891,926 discloses a process for devulcanizing cured rubber into devulcanized rubber that can be recompounded and recured into useful rubber products, and for recovering the devulcanized rubber from the cured rubber. The said process comprises the steps of: (1) heating the cured rubber to a temperature within the range of about 150° C. to about 300° C. under a pressure of at least about 3.4×106 Pascals (34.0 atm) in 2-butanol to devulcanize the cured rubber into the devulcanized rubber to produce a mixture of solid cured rubber, solid devulcanized rubber and a solution of the devulcanized rubber in the 2-butanol; (2) removing the solution of the devulcanized rubber from the solid cured rubber and the solid devulcanized rubber; (3) cooling the solution of the devulcanized rubber in the 2-butanol to a temperature of less than about 100° C.; and (4) separating the devulcanized rubber from the 2-butanol.
U.S. Pat. No. 6,380,269 discloses a process for devulcanizing the surface of reclaimed rubber crumb into surface devulcanized reclaimed rubber crumb that is suitable for recompounding and recuring into high performance rubber products. The said process comprises the steps of: (1) heating the reclaimed rubber crumb to a temperature within the range of about 150° C. to about 300° C. under a pressure of at least about 3.4×106 Pascals (34.0 atm) in the presence of 2-butanol to devulcanize the surface of the rubber crumb thereby producing a slurry of the surface devulcanized reclaimed rubber crumb in the 2-butanol, wherein the reclaimed rubber crumb has a particle size within the range of about 325 mesh to about 20 mesh; and (2) separating the surface devulcanized reclaimed rubber crumb from the 2-butanol.
U.S. Pat. No. 6,416,705 discloses a process and apparatus for devulcanizing cured, or cross-linked, elastomer or various kinds of rubber by the steps of: (1) subdividing them into small particle form; (2) confining the particles of elastomer under high force, as in a screw extruder or the like; and (3) imparting ultrasonic energy to the particles remaining confined to effect devulcanization. Energy is fed to the confined particles transversely of the axis along which they are advancing, and energy from a source is reflected off a portion of the apparatus and back into the treatment zone so as to achieve maximum energy utilization. In particular instances, reflection of energy is achieved by providing opposed, powered ultrasonic horns that resonate in phase with each other. In another embodiment, oppositely directed, resonantly tuned horns are used, with less than all such horns being powered and the remainder being passive or unpowered reflecting horns whose resonant frequency is tuned to that of the powered horn. In one apparatus, the pairs of horns resonate in phase by reason of delay lines interposed between the two power sources. In the other form, the mass of the passive horn is balanced with that of the active horn to achieve in-phase tuning that maximizes reflection of energy.
U.S. Pat. No. 6,479,558 discloses a process and resulting product in which a vulcanized solid particulate, such as vulcanized crumb rubber, has select chemical bonds broken by biotreatment with thermophillic microorganisms selected from naturally occurring isolates derived from hot sulfur springs. The biotreatment of the crumb rubber renders the treated crumb rubber more suitable for use in new rubber formulations. As a result, larger loading levels and sizes of the treated crumb rubber can be used in new rubber mixtures.
U.S. Pat. No. 6,541,526 discloses a mechanical/chemical process and composition for devulcanizing cured rubber that maintains the macromolecules and renders sulfur passive for later re-vulcanization. The said process comprises the steps of: (1) shredding and crushing used rubber; (2) removing metal pieces from shredded and crushed rubber; and (3) adding a modifying composition as the particles of shredded waste rubber are poured between two rollers that further crush the particles. The modifying composition is a mixture of a proton donor that selectively breaks the sulfur bonds and renders the sulfur passive; a metal oxide; an organic acid that builds new bonds between macromolecules for later re-vulcanization; an inhibitor that prevents re-attachment of sulfur radicals with each other before the proton donor attaches itself to the sulfur; and a friction agent that prevents sliding of the waste rubber between the rollers. The particles are subjected to at least ten sets of rollers.
U.S. Pat. No. 6,590,042 discloses a process for reclaiming sulfur-cured, i.e., vulcanized, rubber, by the steps of: (1) combining finely ground scrap vulcanized rubber within a specialized, twin-screw extruder capable of providing strong shearing and just-on-time mixing; (2) adding a reclaiming agent to the extruder; and (3) masticating the rubber scrap and reclaiming agent within the extruder until the rubber scrap is devulcanized. The said patent also discloses a unique composition of reclaiming agent, which preferably includes the following compounds: accelerators, TBBS, ZMBT, MBT, and TMTM; activators, zinc oxide and stearic acid; and zinc salts of fatty acids and sulfur. The reclaimed rubber is suitable for producing high-grade, rubber products without adding a binder, or for combining with fresh rubber compounds to produce high-specification rubber products.
U.S. Pat. No. 6,831,109 discloses a process that provides a modifier for devulcanization of cured elastomer, and especially vulcanized rubber. The said modifier contains a first chemical substance, which is disposed towards on and the formation of an organic cation and amine, and further contains a second chemical substance as promoter of dissociation of the first chemical substance. The said promoter contains a functional group constituting an acceptor of said amine.
U.S. Pat. No. 6,924,319 discloses a process for devulcanizing comminuted scrap rubber of rubber granules, the sulfur bridges on which are broken and activated for revulcanization. The said process comprises the steps of: (1) treating the rubber granules to swell the rubber structure of the granule surface; and (2) mixing the treated rubber granules with a devulcanization formulation, acting mechanically and chemically reductively on the rubber granules, in a heating and cooling mixer combination. The rubber granules and the devulcanization formulation are heated to a temperature of 105-150° C. and subsequently cooled immediately. A devulcanization compound is prepared by mixing the devulcanization product with vulcanization and binding agents so as to coat the rubber granules uniformly with them. The devulcanization compound can also be prepared by coating the swelled rubber granules in layers by admixing vulcanization agents such as accelerators, activators, auxiliary agents, binding agents, oxygen radical donors, and scavengers.
U.S. Pat. No. 6,992,116 discloses a process whose invention is based upon the unexpected discovery that the surface of reclaimed rubber crumb particles can be devulcanized by heating the crumb particles to a temperature of at least about 150° C. under a pressure of at least about 3.4×106 Pascals (34.0 atm) in the presence of 2-butanol. It is further based upon the unexpected discovery that such surface devulcanized rubber crumb particles having a particle size within the range of about 325 mesh to about 20 mesh can be recompounded and recured into high performance rubber products, such as tires, hoses and power transmission belts. This patent more specifically discloses a process for devulcanizing the surface of reclaimed rubber crumb into surface devulcanized reclaimed rubber crumb that is suitable for recompounding and recuring into high performance rubber products. The said process comprises the steps of: (1) heating the reclaimed rubber crumb to a temperature within the range of about 150° C. to about 300° C. under a pressure of at least about 3.4×106 Pascals (34.0 atm) in the presence of 2-butanol to devulcanize the surface of the rubber crumb thereby producing a slurry of the surface devulcanized reclaimed rubber crumb in the 2-butanol, wherein the reclaimed rubber crumb has a particle size within the range of about 325 mesh to about 20 mesh; and (2) separating the surface devulcanized reclaimed rubber crumb from the 2-butanol.